Septic Arthritis

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Also known as: Infectious Arthritis


Infectious arthritis describes the presence in joints of a bacterial, viral or fungal infection. This leads to cartilage damage due to the release of enzymes, the accumulation of fibrin, white blood cells and organisms. There may be the formation of pannus, which is a granulation tissue overgrowth of the cartilage surface, and a loss of glycosaminoglycans which protect the cartilage.

Bacterial arthritis occurs most commonly in food animals, especially young animals, in horses, and more rarely in small animals.

Ports of entry into the joint include:

Trauma: from a penetrating wound, a bite
Extension: from adjacent tissue or bone
Haematogenous: from an umbilical infection, pneumonia, urinary tract infection, endocarditis, dermatitis, periodontal disease, discospondylitis
Iatrogenic: joint injection or surgery

Species Differences




  • Neonatal polyarthritis by:
Streptococcus spp.: via the umbilicus and also involves eyes and meninges
Coliforms: Localise in joints and meninges in severe non-fatal neonatal colibacillosis. The infection may remain as chronic arthritis in larger joints.
  • Infections at any age:
Corynebacterium pyogenes: suppurative arthrtis, often due to a penetrating wound into or close to joints
Mycoplasma spp.: may be responsible for some chronic cases but difficult to prove as hard to isolate, leads to a fibrinous polyarthritis
Chlamydia sp.: severe disease in young calves with a high mortality. The organism can be seen in smears of synovial fluid from swollen joints. The surrounding tissue is oedematous and hyperaemic. It is possibly due to intrauterine infection.
Brucella abortus: uncommonly localises in joints but can occur in carpal bursitis (hygroma) in cattle and bursitis in horses.


  • Arthritis can occur following neonatal infections by:
Actinobacillus equuli
Streptococcus spp.



Clinical Signs

Infectious arthritis usually presents as a shifting lameness in one or more joints.

There will be joint effusion, heat, periarticular swelling, and there may be systemic signs such as fever, depression, anorexia and lymphadenopathy.

The condition usually progresses rapidly.

If a wound is involved with the joint there may be open drainage with leakage of synovial fluid. These animals are usually less painful as it is the pressure of joint capsule distension by fluid that causes the pain.


The diagnosis is made on the basis of the clinical signs and the results of synovial fluid examination. The synovial fluid should be collected before administering antibiotics.

Culture and sensitivity in aerobic and anaerobic media should be performed though up to 50% of cultures will yield false-negative results. Inoculating the fluid into blood culture medium and then blood agar increases the chances of obtaining a positive result.

Gram stain and smear examination of the fluid will reveal a very high number of white blood cells, over 90% neutrophils. Neutrophils may be degenerate with intra and extra-cellular bacteria.

Total protein is usually >4g/dl. The fluid is of low viscosity, high volume and turbid.

Radiography can help in the diagnosis and is essential in providing a baseline of joint anatomy to evaluate any changes that might occur down the line.

Contrast radiography may help in horses to determine if a joint has been entered by a penetrating foreign body.


Antibiotics should be administered for a minimum of 4-6 weeks, initially broad spectrum and then depending on culture and sensitivity results.

Drainage and flushing is mandatory. Usually open drainage is performed via an arthrotomy. This allows debridement and lavage with high volumes of isotonic fluid.

Needle distension and irrigation is less effective as fibrin can occlude the needles.

The joint should be rested, but its range of motion should be preserved through passive range of motion exercises or swimming.

Exercise should be restricted for 3-4 months.

NSAIDs can be administered for pain, corticosteroids are contraindicated in cases of infection.

Chronic cases with irreversible bony damage may require salvage surgery such as arthrodesis, excision arthroplasty or amputation, especially if the pain is uncontrollable.

Septic Arthritis Learning Resources
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Test your knowledge using flashcard type questions
Small Animal Orthopaedics Q&A 15


Pasquini, C. (1999) Tschauner's Guide to Small Animal Clinics Sudz Publishing

Merck and Co (2008) Merck Veterinary Manual Merial

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